You can expect everything that is not by itself (very) expensive. So no Txogitxu and Lobster. (There actually are Menus that include this, but not on the Frances and with a digit more at the "precio" section).
I tend to go with what the waiter recommends or what i know is regional. Of course theres exceptions.
What i seen in Menu del Dia (say in the price range of 10-25€):
- Salads. Mixed, Russian, Tomato...
- Soups. Lentils, Chick Peas, Caldo...
- Fish, cooked, fried, baked, grilled
- Seafood in all varieties (the less expensive like mussels and calamari)
- Meat. From a fried up slice to an honest to god porterhouse steak.
- "interesting parts" of the pig. I think one dish i had was stuffed pig intestines. Actually tasted quite good.
- Ham and Cheese. But thats actually quite rare.
- I guess i seen Paella as well as part of the menu. But i usually don't eat that along the camino.
And usually some kind of desert or coffee. In one place i got lucky and had the waiter replace the postre with a carajillo. Guess that doesnt work everywhere...
edit: pictures are of various different menus i had over different years. Also only the ones i found special.